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Amphiboles are double chain inosilicates, such as
actinolite
☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2,
edenite
NaCa2Mg5(Si7Al)O22(OH)2,
hornblende
☐Ca2(Fe2+4Al)(Si7Al)O22(OH)2,
pargasite
NaCa2(Mg4Al)(Si6Al2)O22(OH)2 and
tremolite
☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22
(OH)2.
Crossite is a discontinued amphibole name for a mineral intermediate between the
riebeckite group and the
glaucophane group.
Clinoamphibole is a generic name for amphiboles crystallising in the monoclinic system.
In the discontinuous branch of the Bowen reaction series amphibole is intermediate
between pyroxene (higher temperature) and
biotite (lower temperature).
Environments:
Amphibole is typical of plutonic igneous environments.
Amphibole is a common but not essential constituent of
rhyolite and
gneiss.
It also may be found in
granite,
quartzolite,
gneiss and
eclogite.
Amphibole is the characteristic mineral of the amphibolite facies;
it is never found in the granulite facies.
Alteration
Ca-Fe amphibole and anorthite to
chlorite, epidote and
quartz
CaFe5Al2Si7O22(OH)2 +
3CaAl2Si2O8 + 4H2O →
Fe5Al2Si3O10(OH)8 +
2Ca2Al3Si3O12(OH) + 4SiO2
(JVW p363)
calcium amphibole, calcite and
quartz to diopside-
hedenbergite,
anorthite,
CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 +
3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 +
2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks
belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.
(DHZ 2A p272)
calcium amphibole, grossular and
quartz to diopside-
hedenbergite, anorthite,
pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 +
Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 +
4Ca(Al2Si2O8) +
(Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks
belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.
(DHZ 2A p272)
calcium-iron amphibole and anorthite to garnet
(grossular and almandine),
clinozoisite and quartz
Ca2Fe3Si8O22(OH)2 + 6Ca(Al2Si2O8) ⇌
4/3Ca3Al2(SiO4)3 + 5/3Fe3Al2(SiO4)3
+ 2Ca2Al3[SiO7][SiO4]O(OH) + 5SiO2
(MM 48.206)
amphibole, chlorite,
paragonite, ilmenite,
quartz and calcite to
garnet, omphacite,
rutile, H2O
and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 +
Mg5Al(AlSi3O10)(OH)8 +
3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 +
9SiO2 + 4CaCO3 →
2(CaMg2Fe3)Al4(SiO4)6 +
4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O +
4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as
the above.
(DHZ 2A p453)
amphibole, clinozoisite,
chlorite, albite,
ilmenite and quartz to
garnet, omphacite,
rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 +
2Ca2Al3[Si2o7][SiO4]O(OH) +
Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 +
4Fe2+Ti4+O3 + 3SiO2 →
2(CaMg2Fe3)Al4(SiO4)6 +
4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.
(DHZ 2A p453)
chlorite (clinochlore), iron-poor
epidote
and SiO2 to amphibole (tschermakite),
anorthite and H2O
3Mg5Al(AlSi3O10)(OH)8 +
6Ca2(Al2Fe3+)[Si2O7][SiO4]O(OH)
+ 7SiO2 →
5☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2
+ 2Ca(Al2Si2O8) + 10H2O
This reaction occurs at a fairly high metamorphic grade.
(DHZ 3 p154)
enstatite-ferrosilite,
diopside-hedenbergite,
albite, anorthite and H2O to
amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 +
NaAlSi3O8
+ Ca(Al2Si2O8) + H2O ⇌
NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 +
4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies, and it is the reason
why amphibole is never found in granulite facies rocks.
(DHZ 2A p139)
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